588 CVR OS PESROLIV AN, 
D is formed there will be a union of C and D to form A and B. 
This reversible reaction is expressed thus :" 
feel 2) orcs (68s 0 DY 
Let the degrees of concentration of the active masses, z. ¢., the 
masses in’ units*volume ot A, B, Cand D be) respectively 
~,9,p and g’. The unit measure of concentration is the gr. 
equiv. per liter. “Let K be the rate of “combination of unit 
masses of A and B, and K’ that of C and D. Now the magni- 
tude of the reaction of A upon B is evidently K pg (for the 
number of impacts of A upon B depends directly on their con- 
centration) and that of C and D is K”. p’ g’. Now when 
equilibrium is established, the reaction of A upon B =the reac- 
tion of C upon D or Kpg—K’ p’ gq’ (the law of mass action). 
Dissociation.— From the study of electrolysis? of salts, Clausius 
was compelled to assume that at least a small portion of the salt 
was dissociated into its positive and negative ions, Na Cl, for 
instance, into the positive ion (Na) and the negative ion (Cl). 
Arrhenius3 found that the osmotic pressure of a dilute solution 
of a salt in water, is that which it should have if a large propor- 
tion of the molecules of the salt are split into two smaller 
molecules (Na Cl for instance, into Na and Cl) and also that this 
change in osmotic pressure is a function of the electric conduc- 
tivity. He was therefore compelled to conclude that practically 
all of the salt may be dissociated in very dilute solutions, but, in 
more concentrated solutions the proportion dissociated becomes 
smaller. It then became evident that this action is reversible 
and is expressed 
+ nets 
Na + Cl 2== NaCl. 
S 
* A simple illustration of a reversible reaction is the evaporation of water in an 
enclosed dish. Particles of water leave the liquid to form the superincumbent layer 
of water gas. As soon as any of the gas is formed it in turn gives back water par- 
ticles to the hquid. Equilibrium is established and there is apparently no farther 
evaporation when the number of particles given off by the liquid equals the number 
of particles returned by the gas. 
? Ges. Abh. (sep. papers) II, 135, 1867. 
3 ZEITS: Phys. Chem., Vol. I, pp. 631 ff. 
